Refrigeration systems are commonly used in all types of transport vehicles for transporting perishable items, such as produce. As is typical in refrigeration and air conditioning systems, such systems include a compressor for compressing a refrigerant that is received by the compressor in a gaseous form and is compressed into a liquid form. This compression heats the refrigerant and the waste heat is convected away from the system by passing the refrigerant through a radiator (condenser) downstream of the compressor. The compressed refrigerant is then passed into an evaporator where it is allowed to expand into the gaseous form. This expansion cools the fluid which draws heat from the environment to produce the desired cooling. The gaseous refrigerant is then returned to the compressor. The amount of cooling is controlled by controlling the speed of the compressor. The refrigeration system attempts to provide and maintain a desired temperature in a “box” or storage volume of the vehicle, which is typically a semi-trailer pulled by a truck but may also be a railroad car pulled by a train engine.
Power for turning the compressor has typically been provided by a dedicated internal combustion engine having its own dedicated fuel supply. The cooling output is controlled by controlling the output of the engine. While providing a straight-forward means for regulating cooling, the dedicated engine has the disadvantage that it adds cost to the refrigeration system and is typically not as efficient as the engine used to power the vehicle itself. It is also a drawback of such prior art systems that maintaining two separate fuel supplies is inconvenient.
Alternatively, in the typical air conditioning system used in passenger vehicles, power for the system is obtained from the vehicle engine. The power is typically taken from the engine by belts and pullies and transmitted directly to the compressor. However, the power provided to the compressor varies with engine speed, which in turn varies with vehicle speed, so the amount of cooling cannot be controlled independently of the desired operation of the vehicle. Heat from the vehicle's cooling system can be used to compensate for over-cooling, but this is energy inefficient. Moreover, there is no mechanism for increasing the cooling if the engine output is too low.
In the context of a marine vehicle refrigeration system, the present inventor solved the problems associated with both the prior art refrigeration and vehicle air conditioning systems by powering a refrigeration system from the engine used for propelling the vehicle through use of a hydraulic transmission system. The hydraulic transmission system included a pump that was coupled directly to the engine. The engine turned the pump which in turn pressurized hydraulic fluid in hydraulic fluid lines that carried the pressurized hydraulic fluid to the remote location of the refrigeration system. A hydraulic motor received the pressurized hydraulic fluid and was caused to turn as a consequence. The system has not been known to function outside of the marine environment, however. In particular, the system has not been known to function in a truck or other land transport vehicle.
It was a particular insight of the present inventor to employ a variable volume pressure compensated pump to pump the hydraulic fluid. It is a characteristic of such pumps that the pressure output of the pump can be optimized or controlled independent of engine speed. As far as is known, the inventor's recognition of the advantage of this type of pump for the purpose of powering a refrigeration system was and continues to be unique.
Refrigeration systems also typically employ a blower for blowing air through the evaporator, to increase the efficiency of conducting heat from the environment to the expanding refrigerant at the evaporator and also for distributing the cooled air throughout the box. Typically, such blowers are directly connected to the compressor, although older units employed electrical power. When connected to the compressor, the blower speed changes with compressor speed, while electrically powered blowers were typically operated at a fixed speed.
Precise temperature control of the entire interior of the box can be critical. For example, while it is necessary to maintain as low a temperature as possible for highly perishable items, it may be critical that the items not be permitted to freeze. It has been found that prior art refrigeration systems for truck use have not been entirely satisfactory in this regard.
Accordingly, there is a need for a hydraulic power unit for a refrigeration system that provides for improved cooling control without the need for a dedicated engine, particularly for use in trucks or other land transport vehicles.